The present disclosure relates to spread-spectrum communications and, in particular, to a system and method for providing a variable-length correlator for spread-spectrum communications.
In typical communications systems, frequency offsets are removed from the signal using a carrier recovery loop. In typical spread-spectrum systems, the signal or error is derived from the output of a fixed-length correlator that correlates the received signal (i.e., the received frequency chips) against the known spreading sequence. Unfortunately, the removal of large frequency offsets using a fixed-length correlator has drawbacks and disadvantages.
For example, in one application the common pilot channel (xe2x80x9cCPICHxe2x80x9d) pilot signal in wideband code division multiple access (xe2x80x9cWCDMAxe2x80x9d) is spread with a spreading code of length 256 and, based on the accuracy of inexpensive oscillators readily available on the market, a frequency offset of up to 10 kHz is to be tracked out. Based on existing techniques in the prior art, the correlator would have a length of the full 256 chips. However, such a loop cannot track out a 10 kHz offset with such a large delay in the loop.
One potential solution is to use a more accurate oscillator, but these parts can be very expensive so it is desirable to use a less accurate part and provide a novel way to remove the larger resulting offset.
Another potential solution is to try to converge with a fixed length 256 correlator. If that doesn""t converge after a given amount of time, the frequency of a numerically controlled oscillator (xe2x80x9cNCOxe2x80x9d) can be stepped up and the loop can try to converge again. This process might be repeated until the NCO""s frequency is stepped close enough to the actual frequency offset so that the loop can track out the remaining offset. However, this solution can be time-consuming, as the receiver has to try a large number of offsets for the NCO. In addition, such a loop will have difficulty tracking any sudden large changes in the frequency offset.
These and other drawbacks and disadvantages of the prior art are addressed by an apparatus and method for providing a variable-length correlator for spread-spectrum communications. The apparatus includes a derotator, a variable-length correlator in signal communication with the derotator, a phase detector in signal communication with the variable-length correlator, a lock detector in signal communication with the phase detector, and a controller in signal communication with the lock detector for controlling the length of the variable-length correlator.
The corresponding method to track out large frequency offsets in spread-spectrum communications includes receiving signal chips, correlating the received signal chips, computing a loop error in accordance with the correlation, setting a new lock status in accordance with the computed loop error and a previous lock status, and setting a new correlator length in accordance with the new lock status and a previous correlator length.
These and other aspects, features and advantages of the present disclosure will become apparent from the following description of exemplary embodiments, which is to be read in connection with the accompanying drawings.